Tectonic Motion in North America

General Comments - Reference Frame

The horizontal components of tectonic motion that have been estimated from
the GSFC analysis of SLR data are plotted as red vectors on this map. The white
vectors represent the motions for the sites as given by the NUVEL-1A geologically
determined plate motion model. The frame within which these motions are placed
is defined mathematically such that, in an integrated sense, there is no uniform
net rotation remaining (called a "no net rotation" frame, or NNR frame). One
should keep in mind that this is a mathematical idealization and that these
motions only approximate the velocity relative to a fixed mantle.

Broad Description

There are 14 SLR stations plotted on the immense area of this map. We will treat these either individually or in groups. We'll work generally from west to east.

Sites West of the Pacific - North America Boundary

These involve four sites, two in Mexico, and two in southern California. At
the southern tip of the Baja Peninsula is Cabo San Lucas. Here SLR estimates
a motion which is 52 mm/yr, in good agreement with that expected from the NNR-NUVEL1A
model for Pacific motion. There is an 8 degree difference in the direction,
an explanation for which is lacking at this time. Further north, at Ensenada,
the SLR estimate of motion agrees very well with Pacific NNR-NUVEL1A motion.

To the northeast of Ensenada, we cross into the United States and we see that
the motions estimated from SLR for the sites at Otay Mountain and Monument Peak
are somewhat slower than what the NNR-NUVEL1A model suggests and differ approximately
10 degrees in direction. This situation is best understood when it is recognized
that the accrual of strike/slip motion of the Pacific plate relative to North
America is distributed over a broad boundary zone rather than a single fault.
In other words, as one travels westward across the boundary of the North America
and Pacific plates, the transition between North American and Pacific motion
is not abrupt, but is gradual. As one gets further onto the Pacific plate, one's
plate motion will grow to full plate rate. The re are several faults to the
west of the Otay Mountain and Monument Peak tracking sites which have an active
component of strike/slip motion, thus these two SLR sites are said to be in
the western edge of the plate boundary de! formation zone.

Remaining Sites in California

There are three remaining sites in California which nominally reside on the
North America plate. The southernmost of these is Mojave and it's SLR estimated
motion is very similar to that expected for North America modeled motion based
on the NNR-NUVEL1A model. Further north, at Owens Valley and Quincy, we begin
to see again, deflected vector motions relative to the model. This is attributed
to a counter-clockwise block rotation of the Sierra Nevada microblock.

Other North American Sites

Working eastward, the motions estimated for Bear Lake, Utah, Platteville, Colorado and McDonald Observatory, Texas all show some slight deviations from the geologic model. These deviations may be due to more localized surface processes.

Along the east coast of the United States are sites in Richmond, Florida,
Greenbelt, Maryland and Westford, Massachusetts. The site in Richmond is periodically
visited by one of the mobile SLR systems. The tracking history from this site
is not all that extensive, hence the somewhat larger uncertainty ellipse. Greenbelt
serves as one of the fiducial sites by which the reference frame is defined.
It has no uncertainty ellipse since both the north and east components of it's
motion are defined by the NNR-NUVEL1A model. The other constraint is defined
by the northward motion of the site at Maui (see the Pacific
Map). The motion of Westford exhibits general North America plate motion.